3D-microstructure analysis of compacted Na- and Cs-montmorillonites with nanofocus X-ray computed tomography and correlation with macroscopic transport properties

Abstract

Microstructural and mass transport properties of compacted Na- and Cs-montmorillonites (Na-MMT and Cs-MMT) with different swelling properties at dry densities of 0.8 and 1.25 Mg m−3 were investigated by combining three-dimensional microstructure analysis using nanofocus X-ray computed tomography (CT), X-ray diffractometry (XRD), and through diffusion measurement of deuterated water (HDO). The X-ray CT and XRD observations indicated that macropores (micrometer- and submicrometer-sized pores) in the dry state of compacted Na-MMT are filled with gel phases, and the grain sizes of clay particles shifted toward smaller values through the saturation and swelling processes. By contrast, no gel phase and no decrease in the grain and pore volumes were observed for saturated Cs-MMT. The De of HDO of Na-MMT was reduced by half compared to that of Cs-MMT owing to the high swelling capacity of Na-MMT and the occupation of macropores with gel phases. The geometrical factors of the macropores, including tortuosity (τ) and geometric constrictivity (δg), in the dry and saturated states of compacted Na-MMT and Cs-MMT were estimated from three-dimensional images of the macropores (and gel phases in saturated Na-MMT) and from the results of their statistical analysis. The estimated geometric factor (δg/τ2) of saturated Cs-MMT was consistent with the corresponding De/εD0 values derived in the HDO diffusion experiment. In the case of Na-MMT, the larger differences between the geometric factors (δg/τ2) evaluated by nanofocus X-ray CT and the De/εD0 values derived by the diffusion tests can be explained by the electrostatic constrictivity factor and the additional geometrical factors in gel phase and interlayer that are smaller than the detection limit of the nanofocus X-ray CT.